1. Field of the Invention
The present invention relates to an aggregate drying system, as well as components used therein, including an improved aggregate dryer and mass flow apparatus. More particularly, the present invention is directed to a system for making asphalt concrete, sometimes referred to as hot-mix asphalt (hereinafter "HMA"), in which heat typically lost in exhaust gases and moisture evaporated in a heater is recovered to dry and preheat the aggregate, comprising rocks, gravel and sand used in making the HMA.
In general, the presently preferred use contemplated for the system of the present invention is to heat and dry aggregate used to make HMA. The aggregate usually contains a substantial amount of moisture which can be reduced using the apparatus of the present invention
The novel components used in the system include an aggregate feed bin having a roller assembly and adjustable proportioning means for proportioning the amount of aggregate being discharged. The feed bin for enhancing mass flow of aggregate can be used not only in the system of the present invention involving HMA, but may also be used as a cold feed bin to improve the mass flow of any aggregate material from the feed bin.
The present invention also relates to an improved heater bin for heating aggregate material, as well as the hollow heat exchange plates used therein.
The components of the present invention, considered in combination or independently, represent improvements over current and prior art equipment used in drying aggregate and for retaining and discharging aggregate from feed bins.
2. Description Of The Prior Art
Along with the entire asphalt paving industry, the inventor has been working for several years to develop systems for efficiently and economically recovering heat from steam or combustion gas used in a primary or secondary heater for HMA or aggregate, particularly where the heater is a direct fired drum mixer where the heat source is a fossil fuel burner. None of the prior art systems, however, have proven to be completely successful in the varying and rapidly changing situations in which they are used. Thus, for example, when the price of fuel is high, one type of system which may be expensive to manufacture may be economical to purchase and operate. However, in times when the cost of fuel is relatively low, the relatively high manufacturing costs and purchase price cannot be justified on a commercial basis.
One prior art system is disclosed in the inventor's U.S. Pat. No. 4,245,915, entitled "Apparatus For Making Asphalt Concrete," and RE 32,206, entitled "Process for Making Asphalt Concrete." These patents relate to a system for indirectly preheating aggregate in a storage bin using steam evolved during a subsequent heating step, typically including heating a mixture of aggregate and binder. Copending U.S. patent application Ser. No. 905,367, filed Sept. 8, 1986, now U.S. Pat. No. 4,784,216, entitled "Heating and/or Drying Apparatus," in which the inventor herein is named as a coinventor, is directed to another system for indirectly heating or preheating aggregate in a storage bin, preferably using as a source of heat, steam generated from a different type of heater compared to the heater disclosed in the above-identified patents.
U.S. Pat. No. 4,375,959 of Powell, entitled "Waste Heat Recovery in Asphalt Mixing Plant," discloses a system for making HMA using a direct fired dryer in which a portion of the heat in the dryer exhaust gases is used to vaporize water in the process of preheating aggregate and thereby recover the heat in the exhaust gases travelling through ducts extending serially through the aggregate cold feed bins. Water injection is used to initiate condensation of water vapor in the exhaust gases.
There are other examples of prior art systems which dry various aggregate materials. However, none of the systems described specifically above or of which the inventor is otherwise aware in the prior art are believed to be as effective as the system of the present invention, in which a portion of combustion gas, and the heat contained therein, is recycled to the heater used in a direct fired heating system. A fuel savings of about 33% and an increase in system production rate of about 33% compared to standard drum mix processes are expected by recycling the exhaust gases from the drum mixer through the cold feed bins to preheat the aggregate and then recycling a portion back to the drum mixer. Not only does the system of the present invention make use of the energy value of moisture or other liquid contained in the material to be dried, but also, the system of the present invention produces substantially no atmospheric pollution and recycles a portion of the heat energy contained in the exhaust gases to the burner, all using components which are cost effective to manufacture, purchase and use.
In addition, enhanced mass flow is obtained using the feed bin of the present invention, whether the feed bin is used as a cold feed bin or modified to contain hollow heating plates to preheat the aggregate. The feeder bin may also be used as a crusher/sizer, if desired, by appropriately adjusting the adjustable proportioning means of the mass flow enhancing means.